In a mobile communication network, the location of a mobile station is stored using the tracking area where the mobile station is located as a unit. The term tracking area corresponds to the term LA (Location Area) or the term RA (Routing Area) used in a 3G network and includes one or more cells.
In the mobile communication network, there may be provided an access gateway device that manages each mobile station and an HLR (Home Location Register) that stores the identifier of the access gateway apparatus.
Each mobile station has two operation modes, i.e., an active mode and an idle mode.
The mobile station in the idle mode, i.e., the mobile station in a state of waiting for a call, selects a cell having the best channel status (quality) and receives a paging channel (PCH) from the cell. Then, the mobile station checks the received paging channel (PCH) to determine whether a flag is set indicating that there is a call arrival to the mobile station. When it is determined that the flag is set, the mobile station accesses the cell (serving cell) via an uplink channel. Then, the mobile station establishes a connection with the serving cell and goes into the active mode. At this moment, the network (or network side) may identify the serving cell where the mobile station is located.
Therefore, when the mobile station is in the idle mode, no connection is established between the network side and the mobile station. Therefore, the network side is not able to identify the serving cell where the mobile station is located. Because of this situation, the network has to transmit the paging channel (PCH) to each cell belonging to the tracking area.
In each tracking area, each cell issues a notification of an identifier of the tracking area, so that the mobile station in the idle mode receives the notification of the identifier of the tracking area (tracking area identifier). By doing in this way, the mobile station can identify the tracking area to which the mobile station belongs.
For example, when the mobile station moves from a first cell into a second cell which is the cell adjacent to the first cell and the tracking area identifier notified to the mobile station when in the second cell is different from the tracking area identifier notified to the mobile station when in the first cell, the mobile station in the second cell accesses the second cell via an uplink channel to update a location registration of the mobile station. The update of the location registration may be referred to as Tracking Area Update (TAU).
More specifically, the mobile station stores and holds the identifier of the location-registered tracking area. The mobile station reselects a cell (cell reselection) while moving. Upon the cell reselection, the mobile station receives broadcast information from the reselected cell and identifies the tracking area identifier included in the received broadcast information. Then, the mobile station determines whether the received new tracking area identifier is different from the tracking area identifier stored and held in the mobile station. When it is determined that the received new tracking area identifier is different from that stored and held in the mobile station, the mobile station determines that the mobile station has moved out from the tracking area where the mobile station was located before and performs the tracking area update (TAU).
However, when it is determined that the received new tracking area identifier is the same as that stored and held in the mobile station, the mobile station determines that the mobile station has not moved out from the tracking area where the mobile station was located before and does not perform the tracking area update (TAU).
As described above, the mobile station performs the reselection process while moving. However, as long as the mobile station determines that the mobile station is located in the same tracking area, the mobile station does not perform the tracking area update (TAU).
Further, the mobile station in the active mode, i.e., the mobile station in communication, does not perform the tracking area update (TAU).    Non Patent Document 1: 3GPP TR 25.813 V1.0.1